2. Description of Related Art
Traditionally head up displays, which may be used in an aircraft, as shown in FIG. 1 of the accompanying drawings, use a convention spherical lens system 1 to generate a collimated display from an image provider such as a cathode ray tube 2. The light rays emanating from the spherical lens system 1 are reflected by a conventional fold mirror 3 through a spherical exit lens system 4 and from there passed to a combiner 5 from which the image is reflected to provide a collimated display to the viewer 6 such as a pilot of an aircraft. Thus with these conventional displays the collimating optics used, that is the spherical lens system 1 and spherical exit lens system 4, are unnecessarily large and bulky, which is undesirable. For example, a thirty degree field of view to be displayed to the viewer 6 may require a six inch diameter exit lens system 4. This means that the physical size of the conventional head up display as shown in FIG. 1, which includes the combiner 5 for overlaying the projected display to be viewed by a viewer 6 looking through the combiner 5, may limit the use of such a head up display in a cockpit area with insufficient space to accommodate such a head up display.
There is thus a need for a projection display which is smaller in size than conventional in order to fit into smaller spaces and thus be more suitable for restricted space environments such as an aircraft cockpit, in a helmet mounted display or a head mounted display.